We have isolated a 70,000 dalton inhibitor of E. coli RNA polymerase. This inhibitor has been partially characterized and seems to act at the termination of the first round of RNA synthesis to prevent further RNA synthesis. Termination does not appear to take place at specific sites on the DNA other than those used in the absence of the inhibitor. BIBLIOGRAPHIC REFERENCE: Enzymatic Degradation of Uric Acid by Uricase-Loaded Human Erythrocytes. Ihler, G., Lantzy, A., Purpura, J., and Glew, R.H., J. Clin. Invest., 56, 595-602 (1975).